Aucanquilcha Volcanic Cluster Magma Plumbing System Architecture and Magma Storage Timescales for the Gordo and Polan Stage Lavas

Aucanquilcha Volcanic Cluster (AVC) is an 11 m.y. volcanic system in the central Andes that is evolving over four distinct stages of activity. Stages include the Aloncha (8-11 Ma), Gordo (4-6 Ma), Polan (2-4 Ma), and Aucanquilcha (<1 Ma) stages. All stages erupted homogeneous andesitic to dacitic composition lavas. The AVCs plutonic evolution begins as a series of magmatic underpinnings that evolve to a larger, single zone of melting, assimilation, storage, and homogenization (MASH). The underpinnings correlate with the Aloncha and Gordo stages of activity, whereas, the MASH zone is cognate with the Polan Stage and Gordo to Polan transition. Here, we refine the chemical evolution of the magma, determine magma storage timescales, and constrain the evolution of the magma plumbing system architecture between 3-6 Ma using plagioclase and pyroxene mineral chemistry. Magma storage timescales are determined using diffusion geospeedometry. Plagioclase and pyroxene phenocrysts exhibit three distinct textures: clear and rounded cores, clear and euhedral cores, and resorbed rims. Additionally, plagioclase contains spongy and sieved cores, and sieved rims. Plagioclase contains three populations: An30-50, An50-60, and An60-87. Orthopyroxene Mg# ranges from 0.22-0.76, while clinopyroxene Mg# is generally higher and more restricted ranging from 0.59-0.76. Detailed core-to-rim transects of oscillatory zoned plagioclase and pyroxene phenocrysts display mineral chemistry and textures suggesting shifts in magma composition. Abundant resorption textures indicate multiple magmatic events and their likely re-equilibration back into the melt after storage. The 3-6 Ma lavas have more homogenous mineral compositions and longer magma storage timescales than the ~6-11 Ma lavas resulting from slower rates of magma intrusion in a thermally weakened, hybridized crust. The longer magma storage timescales indicate a larger, single magma body, fostering the conditions of MASH, rather than multiple, less-evolved pockets of magma during ascent. These data constrain the plumbing system of the magma chamber during the transition from the Gordo to Polán Stage providing a better understanding of the progression into an increased rate of volcanic activity.